1. Field of Invention
The present invention relates to improvements in multiple layer steel (MLS) cylinder head gaskets for automotive internal combustion engines, including gaskets having stopper layers situated adjacent edges of combustion cylinder bores, and particularly for providing enhanced sealing between cylinder bores.
2. Description of the Prior Art
Those skilled in the art will appreciate issues involved in both design and maintenance of a high quality seal between cylinder apertures of a cylinder block in an internal combustion engine and a cylinder head fastened to the block. In recent years, MLS cylinder head gaskets have become a preferred design choice, wherein all (typically at least three) gasket layers have been formed of steel. Beaded exterior layers have generally been fabricated of 301 stainless steel, a relatively robust metal with a high spring rate for meeting requisite performance requirements over useful gasket life. The center layer, also called a xe2x80x9cspacerxe2x80x9d layer, has generally been formed of less expensive metals, such as 409 stainless steel, or in some cases even zinc-plated low carbon steels, for meeting less rigorous requirements. Indeed, the center layer is rarely directly exposed to the harsh combustion gas environment of the engine.
It is desirable that areas immediately adjacent circumferential edges of engine cylinder bore apertures be subject to considerably greater stresses for assuring proper sealing as compared to areas of the gasket radially remote from the apertures. To meet the greater stress requirements at the bore edge areas, spacer layers are conventionally employed in areas that circumferentially surround each cylinder bore. The spacer layers often contain so-called stoppers designed to provide increased sealing pressures around the combustion apertures. In some cases, the stoppers have been formed of fold-over spacer layer metal, wherein the combustion edge of the spacer layer is extended and folded over or under primary sealing layers. In other cases, the stoppers have been formed as yet additional layers separately provided, e.g., discrete annular rings positioned about the aperture boundaries.
In many instances, the balance between a) the provision of desirable stresses and b) the strength of materials employed has been less than satisfactory. In such instances, the stopper layers have given rise to cracking phenomena at the boundaries of associated sealing beads surrounding the aperture boundaries. This has been a particular issue in narrow web regions between cylinder bores. In such small area regions, high stresses on the beads can exceed bead deflection stress capacity. Resultant cold working and associated bead cracking are detrimental to both the performance and longevity of gaskets that may otherwise provide reliable combustion sealing media. An improved resilient stopper and bead structure would be welcomed by the industry; particularly, one having a web area stopper structure capable of alleviating the noted nuisance cracking issues.
Another area for potential improvement involves the trapping of hydrocarbons within a cylinder head. The latter has been identified as a major source of internal combustion emissions. It is generally known that any crevice or interstice within the combustion chamber, including that produced by the interface of a U-shaped stopper ring and its adjacent mated planar sealing bead layers, is a potential source of trapped hydrocarbons, hence of undesireable pollutants or emissions. This invention provides an improvement directed to minimizing internal combustion emissions.
Finally, an additional area for potential improvement involves the non-uniformity, and hence non-linearity, of sealing stresses about the combustion aperture of a gasket. For example, to the extent that boltholes are arranged asymmetrically about the gasket, and that the shapes of the engine block and cylinder head parts are also asymmetric, there is normally a variable sealing stress gradient about the combustion aperture. Moreover, to the extent that it is relatively difficult to design a perfect variable stress gradient into a combustion aperture of a gasket, a conformable stopper would be a practical expedient. Thus, irrespective of the amount of torque applied to any particular bolt, a conformable stopper would act to provide variable stresses more desirably about the combustion aperture.
An MLS combustion cylinder head gasket includes a relatively thick soft metal stopper that provides a square nosed aperture edge adapted to face the flame front in a cylinder head bore. The square nosed edge is designed to eliminate the trapping of hydrocarbons, which has been identified as a major source of internal combustion emissions. The soft metal stopper is preferably formed of copper, although other soft metals capable of withstanding high temperatures may be employed, as well.
In one disclosed embodiment, the stopper is formed of a malleable metal ring, and is formed over and about the circumferential combustion aperture edge of the gasket by a forming die that both shapes and clinches the metal ring respectively about and to the aperture edge. In the described embodiment, the stopper is applied to a center spacer layer of the gasket. The stopper height is initially controlled by appropriately sizing the thickness of the metal ring from which the stopper is formed. Upon bolted securement of the gasket between engine block and cylinder head, however, the soft stopper material will flow into and follow the variable contours between block and head, and with thus xe2x80x9cconformxe2x80x9d to the normally variable spacing between those members.
In one described embodiment, the stopper is adapted to be positioned on an aperture edge of the center spacer layer situated between two full beads of mirror image outer metal layers of the gasket. The compensating aspect of the stopper is particularly realized in the web areas between cylinder bores, i.e., in the narrow areas defined by cylinder bore edges between any two adjacent cylinder bores.